Coatings for lower melting point substrates, such as ABS and other plastics, have thus far comprised either non-reactive one-component coatings, one-component reactive blocked isocyanate systems, or two component, isocyanate-containing coatings. Non-reactive one-component coatings such as lacquers tend to perform poorly in physical testing. One-component reactive blocked-isocyanates have proven expensive to use, and usually act as lacquers, posing problems with compatibility and storage stability. Two-component, isocyanate containing coatings perform well in physical testing; however, such compositions are more expensive, are difficult to process due to their high or rapidly escalating viscosity in use, and are a proven health hazard. For example, two component acrylic/isocyanate coatings have not been accepted widely by the coating industry because of the toxicity of the isocyanates and the short pot life of these coatings.
For coating plastics and other heat sensitive substrates, a low temperature cure coating is desirable. However, low temperature cure gives coatings that are soft, have poor water and chemical resistance and poor durability.
A more durable acrylic and melamine containing coating that cures at a low temperature, for example, below 105° C., was proposed in U.S. Pat. No. 4,554,319A1, to Heaps et al. The Heaps et al. reference discloses low temperature curable surface coating compositions comprising a mixture of a solution of a hydroxy-functional styrene-allyl alcohol copolymer and an aminoplast, preferably a mixed methoxymethyl/butoxymethyl melamine, with a hydroxyl containing resin, such as an acrylic polymer, alkyd or polyester. The Heaps et al. reference touts the durability of the coatings it provides; however, the reference fails to provide a storage-stable one component coating. Further, the coatings provided by the Heaps et al. reference give poor gloss retention when cured at low temperatures of 77° C. and 104° C., and therefore give inferior resistance to acid, which would include acid rain.
In accordance with the present invention, the present inventors have sought to provide a shelf-stable coating, non-isocyanate containing composition, which can be cured at a low temperature for use on heat sensitive substrates, and, further, which does not suffer from the above-mentioned drawbacks.